1 84 THE FROG CHAP. 



and lined by the retina. Into the outer side of this dark 

 chamber is let a transparent window, the cornea ; behind 

 which, and separated from it by a space containing the 

 aqueous humour, is a vertical curtain, the iris, perforated by 

 an aperture, the pupil. Behind the iris and in close contact 

 with it is the lens, and filling the whole of the dark chamber 

 between the lens and iris and the retina is the vitreous 

 humour. 



The whole eye thus has the structure of a camera ob- 

 scura. The cornea, aqueous humour, lens, and vitreous 

 humour are a series of lenses, so arranged that the rays of 

 light from an external object are refracted and brought to a 

 focus on the retina, where they form a greatly diminished 

 and inverted image of the object. The iris is provided with 

 muscles, by means of which the pupil can be enlarged or 

 diminished ; it therefore acts as a diaphragm and regulates 

 the amount of light entering the eye. Attached to the 

 capsule of the lens are delicate muscles, by means of which 

 the lens can be made more or less convex ; in this way the 

 focus of the entire apparatus can be altered according to 

 whether the object viewed is nearer or farther from the eye. 

 This arrangement for accommodation is, however, much 

 less highly developed in the frog than in man and the 

 higher animals, in which the relatively smaller lens is 

 flatter and distinctly biconvex in form (Fig. 57). Thus the 

 various parts of this complicated organ are so adjusted as 

 to bring the images of external objects to an accurate focus 

 on the back part of the interior of the eyeball, i.e., on the 

 retina. 



A vertical section of the retina (Fig. 58) reveals a very 

 complex structure. On its inner face, i.e., the surface in 

 contact with the vitreous humour, is a layer of nerve-fibres 

 (./), formed by the ramifications of the optic nerve, which, 



